1. Field of the Invention
The present invention relates to an image recording material sensitive to radiation; specifically, an image recording material suitable for use as a so-called direct plate-making negative-working lithographic printing plate precursor capable of directly making a plate with a laser from a digital signal of a computer, and a novel compound suitable for use in the image recording material.
2. Description of the Related Art
So far, as a method of forming an image by exposure with a photopolymerizable composition, various kinds of methods are known, such as a method where a recording layer is formed on a support surface and includes a photopolymerizable composition containing an ethylenically unsaturated compound and a photopolymerization initiator, wherein after image-wise exposure to polymerize and cure the ethylenically unsaturated compound of an exposed portion, a non-exposed portion is dissolved and removed to form a cured relief image, and a further method where, after the adhesive strength of a photopolymerizable composition layer (recording layer) to a support is altered by exposure, the support is peeled to form an image, and a further method where an image is formed by making use of the change in toner adhesiveness of a photopolymerizable composition layer due to light.
Examples of photopolymerization initiators that can be used in these methods include benzoin, benzoin alkyl ether, benzyl ketal, benzophenone, anthraquinone, benzyl ketone or Micheler's ketone, all of which are sensitive to short wavelength light with a center in a UV-region of 400 nm or less.
On the other hand, with the recent developments in the image formation techniques, photosensitive materials highly sensitive to light in a visible region are strongly demanded. For example, many photopolymerizable compositions with a sensitivity range extended to about 500 nm are proposed for a laser plate making system employing an oscillation beam at 488 nm of an argon ion laser. Further, photopolymerizable compositions sensitive to lights in the longer wavelength range exceeding 600 nm are actively studied in response to laser plate making systems employing a He—Ne laser or semiconductor laser, and reproduction techniques for full color images.
There is a known photopolymerizable composition which includes an ethylenically unsaturated compound and a photopolymerization initiation system, wherein the photopolymerization initiation system is composed of a cyanine dye having a specific structure and heterocycles linked through a monomethine, trimethine, pentamethine, or heptamethine chain, and a s-triazine derivative having a specific structure (e.g. see Japanese Patent Application Laid-Open (JP-A) No. 58-29803 and JP-A 4-31863). Besides, another photopolymerizable composition containing a polymerization initiator system composed of a squarylium compound having a specific structure and a specific s-triazine compound is proposed (e.g. see JP-A No. 4-106548).
However, it is known that when considering the active radical generation ability of a photopolymerization initiator with respect to light having a wavelength of 500 nm or more, in particular to light having a wavelength exceeding 600 nm, a decrease in photo-excitation energy is usually accompanied by a rapid decrease in sensitivity. All of the photopolymerizable compositions mentioned above, or that have been proposed so far, are insufficient in sensitivity to light in a long-wavelength region as mentioned above. Furthermore, there is a problem in that the polymerization reaction proceeds when the photopolymerizable compositions are handled under a white fluorescent lamp, making photopolymerizable compositions having stable qualities difficult to obtain at present.
In order to overcome the problems of low sensitivity and inconvenient handling properties under white light, a photopolymerizable composition containing an ethylenically unsaturated compound, a particular dye and a photopolymerization initiator (such as a triazine compound) has been proposed (JP-A No. 2000-131837).
However, a lithographic printing plate precursor that uses such a composition in a recording layer hinders the polymerization due to oxygen in the air at the time of a polymerization reaction, resulting in such problems as deteriorated sensitivity or insufficient strength of a formed image portion.
In order to overcome these problems, methods where a protective layer (oxygen-shielding layer) containing a water-soluble polymer is formed on an image recording layer, or where a protective layer containing an inorganic layered compound and a water-soluble polymer is used, are known (JP-A No. 11-38633). These protective layers are excellent in development removability and, owing to the presence of the protective layer, the effect of polymerization hindrance is prevented, the polymerization reaction of the image recording layer is accelerated, the sensitivity is heightened, and the mechanical strength of an image portion can be improved.
For improved productivity in the plate-making operation of photopolymerizing lithographic printing plate precursors that may be developed conveniently, it is important to shorten the time that the exposure process takes. In the exposure process, lithographic printing plate precursors are usually fed as a stack with an interleaf interposed therebetween. Accordingly, the time it takes to remove an interleaf in the exposure step is a cause of inefficiency. In order to improve the efficiency of the exposure step, it is possible to use a stacked body where an interleaf is not interposed between the precursors to omit the interleaf removing step. However, in such a stack, a protective layer surface of a lithographic printing plate precursor is rubbed by an aluminum support of another plate to cause scratches, affecting the polymerization reaction of the image recording layers, resulting in problems such as deteriorated sensitivity and development removability.
For the above reasons, an image recording material such as a lithographic printing plate precursor, high in sensitivity, excellent in development removability and difficult to scratch during handling is desired.